We propose to acquire a state-of-the-art Beckman Optimal (Tm) XL-I analytical ultracentrifuge equipped with both absorbance and Rayleigh interference optics. No Model E or comparable analytical ultracentrifuge is currently available at the University of Massachusetts. The investigator group is composed of eight NIH-funded investigators; seven users are from the University of Massachusetts Medical Center at Worcester and represent three different academic departments. The eighth investigator is located at the nearby University of Massachusetts campus at Amherst Acquisition of a XL-I analytical ultracentrifuge has become necessary for the nine major users to accomplish aims of their NIH-funded research programs. All of the users are engaged in quantitative studies of biological macromolecules and they bring a broad spectrum of backgrounds and interests to the investigation of molecular structure. Because macromolecular structure and behavior Can vary markedly with concentration, solution components, pH, and temperature, it is often essential that different properties be determined under similar solution conditions. Analyses by analytical ultracentrifugation uniquely fulfills this requirement. Areas under investigation by the major users are 1) assembly of yeast histone octamers and the SWI/SNF chromatin remodeling complex, 2) properties of dynamin and dynein subunits, 3) assembly of cooperative Hemoglobins, 4) RecA filament assembly, 5) Nucleotide dependent interactions of G proteins and exchange factors, 6) Paramyxovirus membrane fusion, 7) Structure and regulation of muscle contraction, and 8) analysis of the 3-D structure of chromatin. Cumulatively, these nine research proposals cover the entire range of questions for which the analytical ultracentrifuge is ideally suited to answer, including measurements of sample purity, detection and characterization of conformational changes, determination of subunit stoichiometries, and characterization of assembly and disassembly mechanisms for supramolecular assemblies. The proposed instrument purchase will fulfill acute research needs of NIH-supported investigators at the University of Massachusetts.